Solutions Architect, 10gen

Marc Schwering

#MongoDBDays - @m4rcsch

Replication and Replica Sets

Notes to the presenter Themes for this presentation:

•  Balance between cost and redundancy.

•  Cover the many scenarios which replication would solve and why.

•  Secret sauce of avoiding downtime & data loss

•  If time is short, skip 'Behind the curtain' section

•  If time is very short, skip Operational Considerations also

Agenda

•  Replica Sets Lifecycle

•  Developing with Replica Sets

•  Operational Considerations

Why Replication?

•  How many have faced node failures?

•  How many have been woken up from sleep to do a fail-over(s)?

•  How many have experienced issues due to network latency?

•  Different uses for data –  Normal processing –  Simple analytics

ReplicaSet Lifecycle

Node 1 Node 2

Node 3

Replica Set – Creation

Node 1Secondary

Node 2Secondary

Node 3Primary

Replication

Heartbeat

ReplicationReplica Set – Initialize

Node 1Secondary

Node 2Secondary

Node 3

Heartbeat

Primary Election

Replica Set – Failure

Node 1Secondary

Node 2Primary

Node 3

Replication

Heartbeat

Replica Set – Failover

Node 1Secondary

Node 2Primary

Replication

Heartbeat

Node 3Recovery

Replication

Replica Set – Recovery

Node 1Secondary

Node 2Primary

Replication

Heartbeat

Node 3Secondary

Replication

Replica Set – Recovered

ReplicaSet Roles & Configuration

Node 1Secondary

Node 2Arbiter

Node 3Primary

Heartbeat

ReplicationReplica Set Roles

> conf = { _id : "mySet", members : [ {_id : 0, host : "A”, priority : 3}, {_id : 1, host : "B", priority : 2}, {_id : 2, host : "C”}, {_id : 3, host : "D", hidden : true}, {_id : 4, host : "E", hidden : true, slaveDelay : 3600} ]

} > rs.initiate(conf)

Configuration Options

> conf = { _id : "mySet”, members : [ {_id : 0, host : "A”, priority : 3}, {_id : 1, host : "B", priority : 2}, {_id : 2, host : "C”}, {_id : 3, host : "D", hidden : true}, {_id : 4, host : "E", hidden : true, slaveDelay : 3600} ]

} > rs.initiate(conf)

Configuration Options

Primary DC

Primary DC

> conf = { _id : "mySet”, members : [ {_id : 0, host : "A”, priority : 3}, {_id : 1, host : "B", priority : 2}, {_id : 2, host : "C”}, {_id : 3, host : "D", hidden : true}, {_id : 4, host : "E", hidden : true, slaveDelay : 3600} ]

} > rs.initiate(conf)

Configuration Options

Secondary DC Default Priority = 1

> conf = { _id : "mySet”, members : [ {_id : 0, host : "A”, priority : 3}, {_id : 1, host : "B", priority : 2}, {_id : 2, host : "C”}, {_id : 3, host : "D", hidden : true}, {_id : 4, host : "E", hidden : true, slaveDelay : 3600} ]

} > rs.initiate(conf)

Configuration Options

Analytics node

> conf = { _id : "mySet”, members : [ {_id : 0, host : "A”, priority : 3}, {_id : 1, host : "B", priority : 2}, {_id : 2, host : "C”}, {_id : 3, host : "D", hidden : true}, {_id : 4, host : "E", hidden : true, slaveDelay : 3600} ]

} > rs.initiate(conf)

Configuration Options

Backup node

Developing with Replica Sets

Secondary Secondary

Primary

Client ApplicationDriver

Write

Read

Strong Consistency

Secondary Secondary

Primary

Client ApplicationDriver

Write

Read Read

Delayed Consistency

Write Concern

•  Network acknowledgement

•  Wait for error

•  Wait for journal sync

•  Wait for replication

write

Driver

Primary

apply inmemory

Unacknowledged

Driver

Primary

apply inmemory

getLastError

MongoDB Acknowledged (wait for error)

Driver

Primary

write tojournal

apply inmemory

getLastError

write

j:trueWait for Journal Sync

Driver

Primary

Secondary

getLastError

write

w:2

replicate

apply inmemory

Wait for Replication

Tagging

•  Control where data is written to, and read from

•  Each member can have one or more tags –  tags: {dc: "ny"} –  tags: {dc: "ny",

subnet: "192.168", rack: "row3rk7"}

•  Replica set defines rules for write concerns

•  Rules can change without changing app code

{ _id : "mySet", members : [ {_id : 0, host : "A", tags : {"dc": "ny"}}, {_id : 1, host : "B", tags : {"dc": "ny"}}, {_id : 2, host : "C", tags : {"dc": "sf"}}, {_id : 3, host : "D", tags : {"dc": "sf"}}, {_id : 4, host : "E", tags : {"dc": "cloud"}}], settings : { getLastErrorModes : { allDCs : {"dc" : 3}, someDCs : {"dc" : 2}} } } > db.blogs.insert({...}) > db.runCommand({getLastError : 1, w : "someDCs"})

Tagging Example

Driver

Primary (SF)

Secondary (NY)

getLastError

write

W:allDCs

Secondary (Cloud)

replicate

replicate

apply inmemory

Wait for Replication (Tagging)

Read Preference Modes

•  5 modes –  primary (only) - Default –  primaryPreferred –  secondary –  secondaryPreferred –  Nearest

When more than one node is possible, closest node is used for reads (all modes but primary)

Tagged Read Preference

•  Custom read preferences

•  Control where you read from by (node) tags –  E.g. { "disk": "ssd", "use": "reporting" }

•  Use in conjunction with standard read preferences –  Except primary

Operational Considerations

Maintenance and Upgrade

•  No downtime

•  Rolling upgrade/maintenance –  Start with Secondary –  Primary last

Replica Set – 1 Data Center

•  Single datacenter

•  Single switch & power

•  Points of failure: –  Power –  Network –  Data center –  Two node failure

•  Automatic recovery of single node crash

Datacenter 2

Datacenter

Member 1

Member 2

Member 3

Replica Set – 2 Data Centers

•  Multi data center

•  DR node for safety

•  Can’t do multi data center durable write safely since only 1 node in distant DC

Member 3

Datacenter 2

Member 1

Member 2

Datacenter 1

Replica Set – 3 Data Centers

•  Three data centers

•  Can survive full data center loss

•  Can do w= { dc : 2 } to guarantee write in 2 data centers (with tags)

Datacenter 1Member 1

Member 2

Datacenter 2Member 3

Member 4

Datacenter 3Member 5

Behind the Curtain

Implementation details

•  Heartbeat every 2 seconds –  Times out in 10 seconds

•  Local DB (not replicated) –  system.replset –  oplog.rs •  Capped collection •  Idempotent version of operation stored

> db.replsettest.insert({_id:1,value:1})

{ "ts" : Timestamp(1350539727000, 1), "h" : NumberLong("6375186941486301201"), "op" : "i", "ns" : "test.replsettest", "o" : { "_id" : 1, "value" : 1 } }

> db.replsettest.update({_id:1},{$inc:{value:10}})

{ "ts" : Timestamp(1350539786000, 1), "h" : NumberLong("5484673652472424968"), "op" : "u", "ns" : "test.replsettest", "o2" : { "_id" : 1 }, "o" : { "$set" : { "value" : 11 } } }

Op(erations) Log is idempotent

> db.replsettest.update({},{$set:{name : ”foo”}, false, true})

{ "ts" : Timestamp(1350540395000, 1), "h" : NumberLong("-4727576249368135876"), "op" : "u", "ns" : "test.replsettest", "o2" : { "_id" : 2 }, "o" : { "$set" : { "name" : "foo" } } }

{ "ts" : Timestamp(1350540395000, 2), "h" : NumberLong("-7292949613259260138"), "op" : "u", "ns" : "test.replsettest", "o2" : { "_id" : 3 }, "o" : { "$set" : { "name" : "foo" } } }

{ "ts" : Timestamp(1350540395000, 3), "h" : NumberLong("-1888768148831990635"), "op" : "u", "ns" : "test.replsettest", "o2" : { "_id" : 1 }, "o" : { "$set" : { "name" : "foo" } } }

Single operation can have many entries

Recent improvements

•  Read preference support with sharding –  Drivers too

•  Improved replication over WAN/high-latency networks

•  rs.syncFrom command

•  buildIndexes setting

•  replIndexPrefetch setting

Just Use It

•  Use replica sets

•  Easy to setup –  Try on a single machine

•  Check doc page for RS tutorials –  http://docs.mongodb.org/manual/replication/#tutorials

Solutions Architect, 10gen

Marc Schwering

#MongoDBDays - @m4rcsch

Thank You